The present invention relates to a method for determining the signal quality in optical transmission systems and for determining interfering effects.
In the future, transmission networks will be embodied as optical networks via which data will be transmitted at extremely high bit rates using the wavelength division multiplex method. In this context, extensive transparency of the networks is sought. However, in the required monitoring of the signal quality, it is then no longer possible to ascertain a violation of code rules; for example, in the case of error correcting codes. Therefore, methods are being developed which enable the signal quality to be assessed independently thereof.
In a method described in published patent application DE 195 04 856, amplitude samples are taken asynchronously with respect to the signal clock and the central moments of the sample are calculated therefrom. These are then compared with empirically obtained reference values in order to derive therefrom a statement about the signal quality.
Patent application DE 198 12 078 specifies a further method for determining the signal quality, which makes it possible to make reliable statements about the signal quality. In this method, the outer edges of a histogram which specifies the probability of the occurrence of different voltage values representing the logic states 0 and 1 are evaluated.
This method is developed further in an earlier application DE 199 14 793. In this case, the measuring arrangements allow not only a shift of the thresholds but also a shift of the sampling instants, as a result of which the eye opening can be determined. The determination of the distribution densities of the samples as a function of the amplitude values is effected by deriving a probability distribution which specifies the frequency of the occurrence of one of the two binary states as a function of the sampling threshold.
Attempts likewise are being made to obtain from the histograms knowledge about interfering influences on the transmission link. However, if different interfering effects occur simultaneously, the evaluation of the probability density distribution suffices for identifying and quantifying the effects. In the event of the simultaneous transmission of a number of signals, multichannel effects that occur, such as cross phase modulation (XPM), four wave mixing (FWM), stimulated Raman cross-talk (SRS-XT) and coherent/incoherent cross-talk (XT), all bring about, on statistical average, a Gaussian widening of the probability density distribution of sampling amplitudes and are, thus, indistinguishable.
It is an object of the present invention to specify a method with which the signal quality can be determined as well as the essential transmission properties. Furthermore, the present intention is to identify interfering effects and ascertain the magnitude thereof.